1. Field of the Invention
The present invention relates generally to implantable cardioverter defibrillator systems and more specifically to a semi-automatic atrial and automatic ventricular cardioverter defibrillator.
2. Background of the Invention
Irregular heart rates known as cardiac dysrhythmias generally involve sudden onset of an abnormal or pathologic rhythm disturbance involving the ventricles and atria, the chambers of the heart. Depending upon the dysrhythmia encountered, the symptoms a patient experiences can range from none at all to sudden death. The severe dysrhythmias usually involve at least the ventricles in either ventricular fibrillation or ventricular tachycardia which leads to severe compromise of the pumping capabilities of the heart. Ventricular fibrillation is the leading cause of the classic sudden death syndrome seen in heart disease patients. Due to the nature and severity of ventricular dysrhythmias, implantable automatic systems have been developed to attempt rapid intervention on behalf of the patient and return the ventricular rhythm to a normal condition. These systems are designed for automatic diagnosis of cardiac dysrhythmias and, in response, will automatically treat the dysrhythmia by delivery of a cardioversion or a defibrillation countershock.
For purpose of discussion, cardioversion should be understood to encompass all modes of treatment for all dysrhythmias, atrial or ventricular. However, due to the severe life threatening nature of ventricular fibrillation, the term defibrillation has come into use to define the specific treatment modality used to convert the ventricular fibrillation back into a more benign rhythm. Consequently, the term cardioversion is used for all other forms of cardiac dysrhythmia treatments that converts the dysrhythmia to a more benign rhythm. This use of the term cardioversion as a treatment modality applies equally well to all ventricular tachycardias, the supraventricular tachycardias, atrial flutter and even atrial fibrillation.
Isolated atrial tachycardia or fibrillation is generally not as severe as its ventricular counterpart. Ventricular fibrillation is immediately life threatening, if not reversed within several minutes the patient will die. Although ventricular fibrillation will often involve atrial fibrillation, rapid and automatic treatment is carried out with the primary purpose of treating the ventricular dysrhythmia and secondarily treating the associated atrial fibrillation.
Patients become symptomatic to lesser or greater degrees in the face of onset of atrial tachycardia or atrial fibrillation. Any particular episode of atrial dysrhythmia may lead to relatively severe symptomatology and the patient would want immediate treatment and relief. However, the degree of symptomatology suffered by the patient is dependent upon a number of factors including ventricular response time, ventricular contractility, concurrent and related disease states, diastolic filling time, and the degree of loss of cardiac output due to absence of the atrial preloading of the ventricles known as the atrial kick. All existing implantable cardioverter defibrillators (ICD) systems that are equipped to treat atrial cardioversion do so in an automatic mode. Automatic ICD systems do not take these factors into account and, thus, fail to anticipate that not all episodes of atrial dysrhythmias require immediate electrical intervention.
The patient not suffering any untoward symptomatology or, at least to them, not suffering worse than what the impending electrical shock will make them feel, may wish to avoid electrical cardioversion treatment for some occurrences of atrial dysrhythmia. Consequently, these patients might prefer to be seen and evaluated by a physician prior to undergoing treatment. Automatic ICD's offering atrial cardioversion defibrillation countershock therapy, do so without regard to the patient's clinical condition. The patient will receive a countershock for occurrences of atrial dysrhythmia even if asymptomatic. With today's automatic ICD systems, some patients may forego having an ICD implanted, despite having repeated bouts of atrial dysrhythmia, because of their concern about being suddenly shocked when still alert and asymptomatic.